What is the meaning of # in C#

The name "C sharp" was inspired by musical notation where a sharp indicates that the written note should be made a semitone higher in pitch. This is similar to the language name of C++, where "++" indicates that a variable should be incremented by 1.

Due to technical limitations of display (standard fonts, browsers, etc.)    and the fact that the sharp symbol (♯ music sharp sign) is not present on the standard keyboard, the number sign (# number sign) was chosen to represent the sharp symbol in the written name of the programming language. This convention is reflected in C# Language Specification.

Microsoft has used this # symbol in other languages like F#, J#.

Source : wiki